The present invention relates to low work surface function coatings for high current density dispenser cathodes and more particularly to barium-activated scandium oxide surface coatings for such cathodes, and methods for making same.
High current density dispenser cathodes are widely used as the electron source in display tubes, camera tubes, oscilloscope tubes, klystrons, transmitter tubes and the like. A characteristic of such cathodic structures is that there is a functional separation between the electron emissive surface and a store of emissive material which serves to produce a sufficiently low work function of the emissive surface.
One type of dispenser cathode is a "scandate" cathode, in which the electron emission takes place from the surface of a porous matrix of, for example, tungsten impregnated with a barium-calcium aluminate mixture which is distributed therethrough. In these cathodes, the surface work function is reduced by impregnating or embedding at least the top surface of the metal matrix with an electron emissive material comprised of scandium oxide (Sc.sub.2 O.sub.3). When this is done, the finished product has a lower surface work function and better long term stability, as compared to either uncoated barium impregnated tungsten dispenser cathodes or to dispenser cathodes coated with a mixture of osmium and ruthenium (surface work function 1.80 to 1.85 eV).
Scandium oxide is, however, a semiconductor/insulator and, at high current densities, its resistance to electron current flow causes significant problems when the oxide particles are in or above the range of microns in size. While this problem has been recognized, procedures developed to overcome it have, so far, not proven to reliably produce high quality materials at a relatively reasonable cost. For example, Hasker et al. disclose in U.S. Pat. No. 4,594,220, multistep method which involves sintering a compressed mixture of tungsten and scandium hydride powder previously deposited onto a porous barium and scandium oxide-containing porous tungsten substrate. In another method, disclosed by Hitachi Corporation in U.K. Patent 2,170,950, a combination of metal (preferably tungsten) and scandium oxide is deposited onto an conventional scandate type scandium oxide impregnated cathode surface by sputtering. This method has proven to produce erratic and inconsistent results because of the difficulty in maintaining the correct ratio of metal and scandium oxide in the sputtered layer.
It is therefore the objective of the present invention to provide an improved scandate cathode and a simpler method for making same.